Returns a bundle containing any extra assets provided with the native ad. If a
mediation partner's ads have assets beyond the standard ones (price, store, etc.), they
can be set using the setExtras method in NativeContentAdMapper,
and can be retrieved via this method. For non-mediation flow, it returns an empty
bundle.

Returns the mediation adapter class name. In the case of a mediated ad response,
this is the name of the class that was responsible for performing the ad request and
returning the ad. For non-mediated responses, this value is null.